The invention relates to a fuel injector for use in supplying fuel to a combustion space of a compression ignition internal combustion engine. Such an injector is suitable for use in, for example, a common rail type fuel system.
Conventional fuel injectors of the outwardly opening type include a valve needle which is slidable within a bore provided in a nozzle body and is engageable with a seating, defined by the nozzle body, to control fuel delivery to the cylinder of the associated engine. The valve needle may be arranged within a sleeve member which defines, together with the valve needle, a delivery chamber for fuel, the sleeve member being provided with two axially spaced sets of outlet openings. The sleeve member is biased by means of a compression spring against an enlarged end region of the valve needle, the spring being arranged within a spring chamber.
In use, fuel is delivered to the spring chamber through a supply passage in communication with a source of fuel at high pressure, fuel within the spring chamber flowing into the delivery chamber by means of flats, slots or grooves provided on the valve needle. When the valve needle is moved outwardly from the bore by a first amount, fuel within the delivery chamber is able to flow out through the first set of outlet openings into the engine cylinder or other combustion space, and when the valve needle is moved outwardly from the bore by a greater amount, fuel within the delivery chamber is able to flow out through both sets of outlet openings. Thus, the fuel injection rate or other fuel injection characteristics can be varied, depending on the extent of outward movement of the valve needle.
One problem with conventional fuel injectors of this type is that, upon initial movement of the valve needle away from the seating, fuel within the spring chamber is able to leak into the engine cylinder through a clearance defined between the sleeve member and the nozzle body. Such fuel leakage is undesirable and can lead to a poor fuel spray pattern and poor emissions.
Another problem with such injectors is that, in use, the nozzle body is exposed to very high temperatures which can lead to the breakdown of fuel, thereby causing fuel lacquer to be deposited on the outlet openings provided in the sleeve member. This can result in significant degradation of injector performance and is a particular problem in fuel injectors for which the size of the outlet openings is relatively small.
It is an object of the present invention to provide a fuel injector in which one or more of the aforementioned problems is alleviated or overcome.
According to a first aspect of the present invention there is provided a fuel injector for delivering fuel to a combustion space, the injector comprising a valve needle movable within a bore provided in a nozzle body and engageable with a seating to control the supply of fuel from the bore, the injector further comprising means for permitting fuel to escape from the bore to a low pressure drain upon initial movement of the valve needle away from its seating to reduce fuel leakage into the combustion space.
The invention provides the advantage that fuel leakage into the combustion space is reduced, thereby improving the fuel injection spray pattern and reducing poor emissions.
The fuel injector may be of the outwardly opening type whereby movement of the valve needle outwardly from the bore moves the valve needle away from its seating to permit fuel delivery into the combustion space.
The valve needle may be moveable with a sleeve member, the sleeve member and the bore together defining a clearance through which fuel can flow upon initial movement of the valve needle away from the seating.
The sleeve member may be provided with first and second sets of outlet openings occupying different axial positions on the sleeve member such that, in use, movement of the valve needle away from the seating into a first fuel injecting position causes the first set of outlet openings to be exposed to permit fuel delivery therefrom, and movement of the valve needle away from the seating into a second fuel injecting position causes the first and second sets of outlet openings to be exposed to permit fuel delivery from both sets of outlet openings.
The means for permitting fuel to escape from the bore may conveniently take the form of a return flow passage for fuel in communication with the low pressure drain. The return flow passage may be defined, in part, by a drilling provided in the nozzle body.
The sleeve member may be provided with an annular groove or recess which communicates with the clearance, movement of the valve needle away from its seating causing the annular recess to communicate with the drilling provided in the nozzle body to permit fuel within the clearance to flow to the low pressure drain.
The fuel injector may include a cover member for covering at least a part of the nozzle body, the cover member and the nozzle body together defining at least a part of the return flow passage for fuel.
In an alternative embodiment of the invention, the bore provided in the nozzle body may be provided with an annular groove or recess. The valve needle may be provided with a return flow passage for fuel which communicates with the annular groove when the valve needle is moved away from its seating to permit fuel to escape from the bore to the low pressure drain. The return flow passage may be defined, at least in part, by a drilling provided in the valve needle.
Conveniently, the valve needle may be provided with first and second sets of outlet openings such that movement of the valve needle away from its seating permits fuel delivery through one or both sets of outlet openings depending on the extent of movement of the valve needle away from its seating.
In a further alternative embodiment of the invention, the fuel injector may be of the inwardly opening type. The injector conveniently includes a delivery chamber for fuel, fuel within the delivery chamber being able to flow through one or more outlet opening provided in the nozzle body when the valve needle is moved away from its seating.
Preferably, the return flow passage for fuel is defined, in part, by a drilling provided in the nozzle body.
The injector may include a cover member, the cover member and the nozzle body defining a part of the return flow passage. The cover member is conveniently shaped to provide a restriction to the flow of fuel through the return flow passage. Conveniently, the cover member may be shaped to form a clearance fit with the nozzle body so as to provide the restriction to the flow of fuel.
In an alternative embodiment, the cover member may be provided with an insert member which defines, at least in part, the restriction to the flow of fuel.
In any of the embodiments of the invention, the cover member may include a lip region which is arranged to sealingly engage a cap nut of the fuel injector.
The lip region may define, together with a region of the nozzle body, a part of the return flow passage for fuel.
Each set of outlet openings may include one or more outlet openings.
According to a second aspect of the present invention there is provided a fuel injector for delivering fuel to a combustion space, the injector comprising a valve needle movable within a bore provided in a nozzle body and engageable with a seating to control the supply of fuel from the bore, the injector further comprising a cover member covering at least a part of the nozzle body, the cover member defining, together with the nozzle body, a passage for fuel, whereby, in use, fuel is supplied to the passage to reduce the temperature of the nozzle body.